<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

      div{
        width: 300px;
        height: 50px;
        background-color: rgba(0, 0, 0, 0.3);
        border-radius: 4px;
        padding: 10px;

        position: relative;/*做参照物*/

      }
      input{
        width: 260px;
        /*height: 30px;*/
        border: 0;
        border-radius: 4px;
        padding:10px 20px;
      }
      img{
        position: absolute;
        top: 14px;
        right: 40px;
      }
      p{
        font-size: 12px;
        color: red;
        margin: 0;
      }
    </style>
</head>
<body>
  <div>
    <input type="text" placeholder="请输入用户名" />
    <img src="http://doc.vrd.net.cn/tstore_v1/images/login/yhm.png" alt="">
    <p>用户名不能为空!</p>
  </div>
</body>
</html>